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排序方式: 共有117条查询结果,搜索用时 390 毫秒
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Steven J. Ostro Lance A. M. Benner Christopher Magri Jon D. Giorgini Randy Rose Raymond F. Jurgens Donald K. Yeomans Alice A. Hine Michael C. Nolan Daniel J. Scheeres Stephen B. Broschart Mikko Kaasalainen Jean‐Luc Margot 《Meteoritics & planetary science》2005,40(11):1563-1574
Abstract— We present June 2004 radar images of asteroid 25143 Itokawa (1998 SF36) that improve upon the longitude‐latitude coverage of images obtained in 2001 by Ostro et al. (2004) and use the 2001–2004 data to refine that paper's constraints on Itokawa's shape. The 2004 images, the first of the asteroid's southern side, look distinctly different from the 2001 images, revealing leading edges that are much more curved and rugged than the nearly convex leading edges seen at northern latitudes in 2001. Itokawa is shaped like a slightly asymmetrical, bent, lumpy ellipsoid with dimensions along the principal axes within 10% of 594 times 320 times 288 m. To illustrate the uncertainty space associated with shape reconstruction from images with suboptimal orientational coverage, we present two alternative three‐dimensional models of the object. 相似文献
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Lance B. Morrissey Philippe Janvier Simon J. Braddy John P. Bennett Susan B. Marriott Peter R. Tarrant 《Geology Today》2006,22(2):66-67
Armoured jawless fish, or 'ostracoderms', lived 450–360 million years ago, and display unusual morphologies, unlike any modern fish group. Since they left no living descendants, their mode of swimming has, until recently, remained speculative, although this is a crucial question as the first true pectoral fins evolved within the 'ostracoderms'. The discovery of the oldest-known fish trails, from the Early Devonian (400 million year old) 'Lower Old Red Sandstone' of south-east Wales offers new insights into the swimming behaviour of these early fish, notably the osteostracan 'ostracoderms' (or cephalaspids), whose horseshoe-shaped head and paddle-shaped pectoral fins have remained a functional riddle. 相似文献
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Stephen R. Wing Louis W. Botsford Lance E. Morgan Jennifer M. Diehl Carolyn J. Lundquist 《Estuarine, Coastal and Shelf Science》2003,57(5-6):859-872
We investigated sources of inter-annual variability in larval supply to crab and sea urchin populations at Bodega Head and Point Reyes in northern California. During the spring and summer upwelling seasons of the years 1992 through 1997 we monitored the weekly settlement rates of nine species of crabs and two species of sea urchins. As observed in previous studies, daily values of alongshore windstress, temperature and salinity provided evidence for the poleward flow of relatively warm, low salinity water from south of Point Reyes, an apparent retention zone, during upwelling relaxation events. In years dominated by these events (1992, 1993, 1995 and 1996) we observed that alongshore windstress, temperature and salinity were coherent and temperature was significantly correlated with cancrid crab settlement. During these years the magnitude of cancrid crab settlement and the fraction of cancrid crabs relative to other crab species settling were high. Over four years of concurrent sampling there was consistently greater cancrid crab settlement at the Point Reyes site, within the retention zone, than at Bodega Head. Settlement of non-cancrid crabs (porcellanids, grapsids, pagurids and majids) was not as closely linked to intra-annual patterns of upwelling and relaxation, possibly due to the shorter seasonal availability of larvae allowing for the influence of fewer relaxation events. Settlement of this group among years was positively correlated with environmental indicators of strong seasonal upwelling; high salinity, Bakun upwelling index and low temperature. Sea urchin settlement events were observed in June and July of 1992, 1994 and 1997 during warming periods when salinity and temperature were increasing and alongshore windstress was low. Across the six years of the study, we found that cancrid crab larvae had a more even seasonal availability than larvae of non-cancrid species, which settled in greatest numbers during the early portion of the upwelling season. Sea urchins settled in greatest numbers during the later part of the upwelling season. Together these patterns demonstrate the taxon-specific way that inter-annual variability in larval supply is forced by the coincidence of larval availability with favorable physical transport mechanisms. 相似文献
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The mid-Cretaceous White Creek batholith in southeast BritishColumbia is a zoned pluton ranging from quartz monzodioriteon the margin, to hornblende-and biotite-bearing granodioritetowards the interior of the batholith, which are in turn crosscutby two-mica granite. This range in rock type is similar to therange displayed by Mesozoic granitoid suites found in the Cordilleraninterior of western North America. The lithological zones inthe White Creek batholith correlate with distinet jumps in majorelement, trace element, and isotopic compositions, and indicatethat several pulses of magma were emplaced within the WhiteCreek magma chamber. The hornblende-and biotite-bearing granitoidsare metaluminous to weakly peraluminous, have strong light rareearth element (LREE) enrichment, and small negative Eu anomalies.These granitoids have initial Sr ranging from +32 to +84 (87Sr/86SrTfrom 0.7069 to 0.7106), initial Nd ranging from 5 to10, and initial 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pbranging from 18.3 to 18.7, 15.58 to 15.65, and 38.3 to 39.0,respectively. The two-mica granites and associated aplites arestrongly peraluminous, and show only moderate LREE enrichmentand strong negative Eu anomalies. These granites have Sr rangingfrom +174 to + 436 (87Sr/86SrT from 0.7169 to 0.7354), Nd rangingfrom 12 to 16, and more radiogenic initial Pbisotope ratios than the hornblende-and biotite-bearing granitoids. Oxygen, Sr, Pb, and Nd isotopes, REE modelling, and phase equilibriumconstraints are consistent with crustal anatexis of Precambrianbasement gneisses and Proterozoic metapelites exposed in southeastBritish Columbia, the product being the hornblende-biotite granitoidsand two-mica granites, respectively. The sequence of intrusionin the White Creek batholith constrains the melting sequence.A zone of anatexis proceeded upwards through the crust, firstmelting basement gneisses then melting overlying metapelites.A model for basaltic magmatic underplating as a primary causeof anatexis of the crust during the mid-Cretaceous magmaticepisode is difficult to reconcile with the absence of earlyCretaceous basalt in the southern Canadian Cordillera. A muchmore likely petrogenetic model is that crustal anatexis wasprobably a response to crustal thickening in association withterrane accretion and collision along the western margin ofthe North American continent. 相似文献
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In this paper, we compare and contrast a Bayesian spatially varying coefficient process (SVCP) model with a geographically
weighted regression (GWR) model for the estimation of the potentially spatially varying regression effects of alcohol outlets
and illegal drug activity on violent crime in Houston, Texas. In addition, we focus on the inherent coefficient shrinkage
properties of the Bayesian SVCP model as a way to address increased coefficient variance that follows from collinearity in
GWR models. We outline the advantages of the Bayesian model in terms of reducing inflated coefficient variance, enhanced model
flexibility, and more formal measuring of model uncertainty for prediction. We find spatially varying effects for alcohol
outlets and drug violations, but the amount of variation depends on the type of model used. For the Bayesian model, this variation
is controllable through the amount of prior influence placed on the variance of the coefficients. For example, the spatial
pattern of coefficients is similar for the GWR and Bayesian models when a relatively large prior variance is used in the Bayesian
model.
相似文献
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We examined post-spill trends (1989-1998) of marine bird populations in Prince William Sound (PWS) following the Exxon Valdez oil spill (EVOS) to evaluate recovery of injured taxa. Two criteria were employed. First, we examined population trends of injured taxa only in the oiled area of PWS using regression models. Second, we examined population trends of injured taxa in the oiled area relative to the unoiled area using homogeneity of the slopes tests. We considered a population recovering if there was a positive trend using either criteria. We considered a population not recovering if there was no trend using either criteria or a negative trend in the oiled area. A significant negative trend in the oiled area relative to the unoiled area was considered a continuing and increasing effect. Most taxa for which injury was previously demonstrated were not recovering and some taxa showed evidence of increasing effects nine years after the oil spill. Four taxa (loons Gavia spp, Harlequin Duck Histrionicus histrionicus, Bufflehead Bucephala spp, and North-western Crow Corvus caurinus) showed weak to very weak evidence of recovery. None of these taxa showed positive trends in both winter and summer. Nine taxa (grebes Podiceps spp, cormorants Phalacrocorax spp, Black Oystercatcher Haematopus bachmani, Mew Gull Larus canus, Glaucous-winged Gull Larus glaucescens, terns Sterna spp, murres Uria spp, Pigeon Guillemot Cepphus columba, and murrelets Brachyramphus spp) showed no evidence of recovery during summer or winter. Four taxa (scoters Melanitta spp, mergansers Mergus spp, goldeneyes Bucephala spp, and Black-legged Kittiwaka Rissa tridactyla) showed evidence of continuing, increasing effects. We showed evidence of slow recovery, lack of recovery, and divergent population trends in many taxa which utilize shoreline and nearshore habitats where oil is likely to persist. Potential lingering spill effects and natural variability appear to be acting in concert in delaying recovery of many PWS bird populations. 相似文献
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Anil Misra Lance A. Roberts Steven M. Levorson 《Geotechnical and Geological Engineering》2007,25(1):65-77
Load displacement analysis of drilled shafts can be accomplished by utilizing the “t-z” method, which models soil resistance
along the length and tip of the drilled shaft as a series of springs. For non-linear soil springs, the governing differential
equation that describes the soil-structure interaction may be discretized into a set of algebraic equations based upon finite
difference methods. This system of algebraic equations may be solved to determine the load–displacement behavior of the drilled
shaft when subjected to compression or pullout. By combining the finite difference method with Monte Carlo simulation techniques, a probabilistic load–displacement analysis can be conducted. The probabilistic analysis is advantageous
compared to standard factor of safety design because uncertainties with the shaft–soil interface and tip properties can be
independently quantified. This paper presents a reliability analysis of drilled shaft behavior by combining the finite difference
technique for analyzing non-linear load–displacement behavior with Monte Carlo simulation method. As a result we develop probabilistic relationships for drilled shaft design for both total stress (undrained)
and effective stress (drained) parameters. The results are presented in the form of factor of safety or resistance factors
suitable for serviceability design of drilled shafts. 相似文献